lockd: Remove unnecessary memset()

Since commit 103cc1fa ("SUNRPC: Parametrize how much of argsize
should be zeroed") (and possibly long before that, even) all of the
memory underlying rqstp->rq_argp is zeroed already. There's no need
for the memset() in nlm4svc_decode_shareargs().

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: NeilBrown <neilb@suse.de>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>